Add vertex without subdividing edge then vertex slide?
First off you can use the knife tool to add only one vertex.
All you have to do is LMB  on one side of the edge, then LMB  on the other side, and press Enter or space to confirm.





There is another, potently faster, way of creating a new vertex. That is done by selecting a vertex on the end of the edge you want to subdivided and pressing AltD then G. That will run the Rip Edge tool then edge slide to move the vertex to where you want it to be. Because that tool automatically starts the translation on the new vertex you only have to press G once to get to vertex slide mode.





However using any of these methods will create Ngons, and in most cases really mess up your topology. So you should be very careful how and when you just add a new vertex in the middle of your mesh.